Cathodic protection of underground metals



June 17, 1952 H. A. ROBINSON 2,601,214

CATHODIC PROTECTION OF' UNDERGROUND METALS Filed May 2, 1947 MdneJ/Um @noo/e Ear//J INVEN TOR.

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ATTORNEYS Patented June 17, 1952 ROUND METALS Harold A. Robinson, Midland, Mich., 'assgnorto The Dow Chemical Company, Midland, Mich.. a corporation of Delaware Application May 2, 1947, Serial No. 7455451 8 Claims.

are buried in the earth near the structure and connected to it by electrical conductors. 'Ihe resulting flow of current maintains the structure cathodic with respect to the soil and greatly minimizes its corrosion.

Since magnesium tends to corrode uselessly when in direct contact with some types of soils, it is customary, when using this metal as a sacricial anode, to bury it in a prepared bed or backfill designed to controlthe chemical nature of the anode environment. Backlls of lime or gypsum, for example, have been used for this purpose.

The chemical requirements of an ideal backfill for magnesium anodes are various. In use, the material should have good electrolytic conductivity and yet should not be so soluble as to be leached away. It should be capable of wetting easily in ground waters, and should retain moisture during dry spells so as to avoid loss of con ductivity. In order to secure maximum current' eiiiciency from the magnesium, the backfll should tend to minimize localized, i. e. useless, corrosion of the metal. On the other hand, it should promote uniformity of attack during the useful or current-producing consumption of the magnesium. In addition, it must not polarize the anode unduly or form impervious coatings on it. Few if any compositions which even approximately lill these specifications have been available heretofore. A

It has now been found that backlls consisting of a water-pervious bed of magnesium sulflt'e, alone or admixed with other non-acidic solid granular materials, satisfy in high degree the various requirements mentioned.

The invention, then, consists in cathodic protection systems and backfill compositions utiliing magnesium sulflte as hereinafter set forth and claimed, all with reference to the accompanying drawings, in which Fig. 1 is a schematic vertical section showing the manner of using magnesium sulte as an anode backfill in the galvanic protection of a buried pipeline;

Fig. 2 is a similar vertical section through an alternative type of anode bed; and

`pervious mass.

ZOLL-197) Fig. y3 is a Vertical section through a packaged anode.

Magnesium sullite is sparingly soluble in water (about 1.3 percent), forming a saturated solution having a pI-I value of about 9.2 and a specific resistivity of about 110 ohms per cubic centimeter. It wets rapidly but does not dry out readily. In aqueous solutions of magnesium sulte used as anolytes for magnesium anodes, the presence of the magnesium ion tends to minimize galvanic action between the metallic magnesium of the "anode and iron or other cathodic impurities which may be present in the bed or may become exposed as the anode is consumed. Likewise, in such solutions, the sulfite ion acts to reduce the concentration of free oxygen, thereby limiting oxidative attack of the anode. All these factors operate to permit continuous uniform consumption of magnesium anodes discharging in magnesium sulte backlls. The current enieiencies obtained are exceptionally high, being sometimes percent or more.

The magnesium sulite employed in the invention may be the anhydrous or more conveniently one of the hydrated forms. In use, since water is invariably present, the suliite is probably largely in the hexahydrated state. A convenient product is made by nearly neutralizing an aqueous slurry of magnesium hydroxide with gaseous sulfundioxide, separating the hydrous magnesium sulte precipitate thus formed, and drying to a powder or granular form. The presence of unconverted magnesium' hydroxide in the precipitate is desirable.

In practice, the magnesium su'lfite may be used as the 'sole constituent of 'a magnesium anode backill or may be mixed with other non-acidic solid granular materials s to make a water- Typical of s'u'ch substances 'are coarse sand, gravel, crushed washed coke, and `other electrolytically inert iillers. Granular magnesium hydroxide and magnesium oxide may be similarly used, and have the desirable eifect of assisting in maintaining alkalinity in the anolyte. Powdered or granular bentonite (montmorillonite, a sodium-type base-exchange volcanic clay) is also advantageous in that it is swellable in water and tends to retard leaching of the magnesium sulte.

In such mixed backlls, the proportions of ingredients need not be controlled with precision, although it is desirable that the magnesium suliite represent a major proportion of the solid anode medium. Preferred backlls also contain a minor proportion of bentonite, with or without the further addition of a lesser proportion of magnesium hydroxide. A particularly satisfactory mixture is magnesium sulfte hexahydrate 72 percent by Weight, magnesium hydroxide 8 percent, and bentonite 20 percent. In general, to maintain permeability of the backll toward water, the ingredients should initially be in fairly coarse granular form.

A typical installation for cathodic protection is shown in Fig. 1, in Which a steel pipeline 4 buried in the earth is being protected. consumable galvanic anode 5 is an elongated cylindrical body of magnesium provided with a central iron core 6 which terminates in an electrical connector 1. As shown, the anode 5 is buried in the earth near the pipeline, with the core 6 being connected electrically to the line by a conductor 8. A bed of coarse granular hydrous magnesium sulte backll 9 surrounds the anode and is in firm contact with it and with the earth.

In making the installation, a suitable hole is dug and the anode is lowered in place, after which the backll is tamped around it. Alternatively, the backll may be slurried in water and poured around the anode. The electrical conductor to the pipeline is then installed and buried.

A similar installation is shown in Fig. 2, except that the backll in this case contains bentonite and magnesium hydroxide in addition to the magnesium sulfite.

An optional method of installation, which is particularly convenient under some conditions, involves the packaged anode illustrated in Fig. 3. In this case, a magnesium anode 5 provided with electrical connector 1 is centered in a waterpermeable container, such as a paper carton I0. The space between the anode and the container Walls is then lled with a porous mass 9 of magnesium sulte backll. In installing this anode, it is necessary only to dig a hole the size of the carton l0, insert the entire package, tamp the earth around it, and make the necessary electrical connection. The start of electrolytic action may be accelerated by wetting the package thoroughly at the time of burying it.

Infield use of the invention, the number and size of anodes required to secure effective cathodic protection of a given pipeline or other structure is determined by well-known engineering principles, due allowance being made for the high current efficiency obtained with the magnesium sulte backiill. The quantity of backfill needed for each anode issimilarly determined.

While the invention has been described as useful in, the cathodic protection of underground ferrous metal structures, it is applicable in protecting underground structures of any corrodible metal cathodic to magnesium. The sacrificial anodes may be made either of magnesium or of a magnesium-base alloy, both being comprehended by the term magnesium metal as used in the claims.

The

What is claimed is:

1. In combination with an underground structure of a corrodible metal cathodic to magnesium, a cathodic protection system comprising a magnesium metal anode buried in the earth near the structure and electrically connected thereto, such anode being surrounded by and in intimate contact with a bed of non-acidic solid granular material containing a major proportion of magnesium sulte.

2. In combination with an underground ferrous metal structure, a cathodic protection system comprising a magnesium metal anode buried in the earth near the structure and electrically connected thereto, such anode being surrounded by and in intimate contact with a bed of non-acidic solid granular material containing a major proportion of magnesium sulte.

3. A system according to claim 2 wherein the bed consists of magnesium sulte.

4. A system according to claim 2 wherein the bed consists essentially of a major proportion of magnesium sulte and a minor proportion of bentonite'.

5. A system according to claim 4 wherein the bed also contains a minor proportion of magnesium hydroxide.

6 A packaged anode for use in cathodic protection systems comprising a water-permeable container having therein a magnesium metal anode provided with means for connecting an electrical conductor thereto, such anode being surrounded by and in intimate contact with a mass of non-acidic solid granular material containing a major proportion of magnesium sulte.

7. A composition useful as an anode medium in cathodic protection and consisting essentially of a granular mixture of a major proportion of magnesium sulte and a minor proportion of bentonite.

8. A composition useful as an anode medium in cathodic protection consisting of a granular mixture of a major proportion of magnesium sulflte, and minor proportions of bentonite and of magnesium hydroxide.

HAROLD A. ROBINSON.

REFERENCES CTED The following references are of record in the le of this patent:

FOREIGN PATENTS Country Date Great Britain I Dec. rI, 1928 OTHER REFERENCES Number pre-print 

1. IN COMBINATION WITH AN UNDERGROUND STRUCTURE OF A CORRODIBLE METAL CATHODIC TO MAGNESIUM, A CATHODIC PROTECTION SYSTEM COMPRISING A MAGNESIUM METAL ANODE BURIED IN THE EARTH NEAR THE STRUCTURE AND ELECTRICALLY CONNECTED THERETO, SUCH ANODE BEING SURROUNDED BY AND IN INTIMATE CONTACT WITH A BED OF NON-ACIDIC SOLID GRANULAR MATERIAL CONTAINING A MAJOR PROPORTION OF MAGNESIUM SULFITE. 